1. Field of the Invention
The present invention relates to the selective removal of more highly unsaturated compounds from mixtures of unsaturated compounds. More particularly the invention is concerned with the selective hydrogenation of acetylenic compounds from mixtures with dienes, such as 1,3-butadiene. The invention provides a novel process for the selective hydrogenation of acetylenes in admixture with other unsaturated compounds.
2. Related Information
Crude butadiene streams contain undesired multi-unsaturated impurities such as vinyl acetylene, ethyl acetylene, methyl acetylene, etc. These impurities need to be removed to produce high quality 1,3-butadiene product. Although the acetylenes are more reactive in the hydrogenation, 1,3-butadiene is not inert under the selective hydrogenation condition, especially as the conversion of acetylenes approaches to near completion. The concentration of vinyl acetylene in the steam cracked crude butadiene stream is generally much higher than the concentration of ethyl acetylene. The reactivity of vinyl acetylene toward selective hydrogenation is higher than ethyl acetylene.
Complete or near complete recovery of 1,3-butadiene and complete removal of both vinyl acetylene and ethyl acetylene is highly desired. However, it is not possible to accomplish this objective in the current commercial processes. In current commercial practice, the selective hydrogenation is carried out in either fixed bed process or catalytic distillation process. Each has its own advantages. The fixed bed unit is easier and cheaper to operate, construct, replace spent catalyst with new catalyst, and regenerate spent catalyst. A catalytic distillation unit generally has higher recovery of 1,3-butadiene and longer catalyst cycle time. But it has higher cost for the catalyst loading and the deactivated catalyst needs to be removed from the distillation column instead of in-situ regeneration. Also catalytic distillation operation has one less independent process variable than fixed bed operation because the temperature is a function of pressure and composition of the materials in the catalytic distillation column.
Supported copper catalysts and palladium catalysts have been preferred catalysts in cleaning up acetylenic impurities in olefin streams by selective hydrogenation.
In general, the palladium catalysts are very active compared with the copper catalysts for selective hydrogenation of acetylenic compounds in the olefinic steams, but have lower selectivity for the acetylenes than copper-based catalysts. But the palladium catalysts exhibit low selectivity for retaining diolefins, such as 1,3-butadiene, when one is trying to remove high concentrations (>2000 ppm) of total alkynes to less than about 500 ppm total alkynes in the streams, especially when the acetylenes are reduced to less than 200 ppm. The non selectivity of palladium catalysts is not desirable in commercial practice, because it results in a loss of 1,3-butadiene. To improve olefin selectivity of palladium catalysts, silver or gold has been added to palladium catalysts in minor amounts as modifier.
On the other hand, the copper catalysts selectively hydrogenate acetylenic compounds without substantial hydrogenation of the olefins and diolefins (designated herein as selectivity for retaining olefins). But the activity of copper catalysts is low and the catalyst cycle time is undesirably short for the feed streams, containing higher than about 2000 ppm total alkynes due to fast deactivation caused by the deposition of polymeric material on the catalyst surface. Even though the hydrogenation may be carried out in liquid phase, some of the polymers deposited on the copper catalyst have little solubility in the liquid product stream under selective hydrogenation conditions. Due to these two reasons, the copper catalysts need improvement for the selective hydrogenation of the mixed olefin feeds, which contain relatively high concentration of total alkynes.
U.S. Pat. No. 4,533,779 disclosed palladium/gold catalysts supported on supports such as alumina (1 to 100 m2/g) for selective hydrogenation of acetylenic compounds. The contents of palladium and gold in the catalysts were in the range of 0.03 to 1 weight % and 0.003 to 0.3 weight %, respectively.
U.S. Pat. No. 4,831,200 disclosed the process for the selective hydrogenation of alkynes in olefin streams such as mixtures with 1,3-butadiene. The selective hydrogenation was carried out in two steps in sequence. In the first step, the hydrocarbon feed was passed at least partially in liquid phase with hydrogen over the palladium catalyst such as that disclosed in U.S. Pat. No. 4,533,779 discussed above. In the second step, the product stream from the first step was passed again at least partially in liquid phase with hydrogen over the copper catalyst such as that disclosed in U.S. Pat. Nos. 4,493,906 and 4,440,956 discussed above to produce significantly reduced alkyne concentration in the final product stream.
It is an advantage of the present invention that it improves the two step process by the combining a fixed bed selective hydrogenation and a catalytic distillation selective hydrogenation in a specific sequence to take advantage of the best characteristics of disparate systems for selective hydrogenation of C4 acetylenes in a crude butadiene stream.